Variable displacement type gear pump

ABSTRACT

In a variable displacement type gear pump, each of a main gear pump portion and a sub gear pump portion has a drive gear, a driven gear engaging with the drive gear, and an urging portion applying radial load to the drive and driven gears. A bypass passage returns fluid in a discharge-side space of the sub gear pump portion to a suction passage. An opening and closing valve opens the bypass passage in a small displacement operational state. A drive shaft supports the main drive gear and the sub drive gear. A driven shaft supports the main driven gear and the sub driven gear. The drive gears and the driven gears are rotated in accordance with a rotation of the drive shaft. The drive and driven shafts transmit the radial load in the main gear pump portion to the sub gear pump portion in the small displacement operational state.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application Nos. 2008-030925 filed Feb. 12, 2008 and 2007-207331 filed Aug. 9, 2007.

BACKGROUND OF THE INVENTION

The present invention relates to a variable displacement type gear pump. Specifically, the variable displacement type gear pump has a plurality of gear mechanisms including a drive gear and a driven gear, and the plurality of gear mechanisms is accommodated in a plurality of gear chambers which is independently formed.

Generally, a gear pump includes a gear mechanism having a drive gear and a driven gear therein. The gear pump introduces fluid from the outside, pressurizes the fluid through the gear mechanism, and discharges the fluid to the outside. In case that hydraulic oil is utilized as the fluid for the gear pump, the gear pump is capable of operating a hydraulic equipment located in a hydraulic circuit. The gear pump has a simple structure, compared to other type pumps. Therefore, the operation and the maintenance are easily performed, and further, the manufacturing cost is low. In addition, the gear pump is not easily affected by foreign matters in the fluid. The gear pump is appropriate for downsizing and reducing weight. Thus, the gear pump is commonly utilized, for example, as a hydraulic pump in an industrial vehicle such as a forklift, and is operated by a combustion engine for running the industrial vehicle.

The discharge flow rate of the gear pump is determined in accordance with the rotational speed of the gear pump, and it is difficult to change the flow rate irrespective to the rotational speed. When the gear pump is operated more than demands, the gear pump generates excessive flow rate, and performs excessive work as a gear pump. A variable displacement type gear pump with a plurality of gear mechanisms has been proposed for achieving variable discharge displacement. The variable displacement type gear pump is shifted between two states. That is, in one state, a specific gear mechanism in the plural gear mechanisms discharges pressurized fluid to the outside, and in other state, the fluid is returned from the specific gear mechanism to the suction side, thereby achieving the discharge displacement.

As a background art related to the variable displacement type gear pump, Japanese Unexamined Patent Publication No. 07-332254 discloses a double gear pump or a double motor. As shown in FIG. 7, the double gear pump or the double motor 100 (hereinafter referred to merely as a gear pump) has gear mechanisms Pa, Pb. The gear mechanism Pb is formed with a drive gear 104 and a driven gear 107 engaging with each other. The gear mechanism Pa is formed with a drive gear 108 and a driven gear 109 engaging with each other. A front cover 101, a housing 102, and a rear cover 103 form a casing so as to accommodate the gear mechanisms Pa, Pb. A partition wall 2W is formed to separate the gear mechanisms Pa, Pb at the middle portion of the housing 102. The gear 104 is formed integrally with an input/output shaft. The gear 107 is formed integrally with a driven shaft. The gear 108 is supported through a bush 106 by the casing coaxially with the gear 104. The axial end of the gear 104 is screwed into the gear 108 so as to be fixed thereto. The gear 109 is supported through an another bush 106 by the casing coaxially with the gear 107. The gear 109 is formed independently of the gear 107, and the gears 107, 109 are independently driven by the gears 104, 108, respectively.

The positions of the teeth of the gears 104, 108 do not generally coincide with each other, because the gear 104 is fixedly screwed into the gear 108. The gear 109 is formed independently of the gear 107, and the positioning process to engage the gears 107, 109 to the gears 104, 108 is easily performed during assembling.

FIG. 8 shows a hydraulic circuit as an example for incorporating therein the gear pump 100 of the above reference utilized as a variable displacement type gear pump. When the gear pump 100 is operated as a variable displacement type gear pump in large displacement state, or, in 100-percent displacement state, a bypass passage L8 is shut off by a switching valve HV. When a motor 120 is driven by electric power from a power source 121, the pump mechanisms Pa, Pb pump up oil in a storage tank 124 through circuits L1, L2, respectively. The oil pumped up by the pump mechanism Pa flows through circuits L4, L6, a check valve CV, a circuit L5, and then joins the oil in the pump mechanism Pb in circuits L3, L7 to be delivered to an actuator 123 (a cylinder 122).

In small displacement operation, the bypass passage L8 communicates through the switching valve HV. Only the oil pumped up by the pump mechanism Pb is delivered to the actuator 123 through the circuits L3, L7. The oil pumped up by the pump mechanism Pa is returned to the supply side of the gear pump 100, or, upstream of the gear pump 100, through the circuit L4 and the bypass passage L8. In the small displacement operation, the oil discharged from the pump mechanism Pa is returned to the suction side through the bypass passage L8. Thereby the pressure at the discharge side of the pump mechanism Pa is lower than the pressure in the large displacement operation. The pressure of the oil discharged from the pump mechanism Pb is higher than the pressure at the discharge side of the pump mechanism Pa, and does not flow through the check valve CV.

In a general gear pump, oil is trapped in a space (hereinafter referred to as a trap region) between teeth of the engaging gears at the gear engaging portion of each pump mechanism. In the trap region, the pressure of the oil is exceedingly increased due to the engagement of the teeth of the gears. When the pressure of the oil is increased in the trap region, urging force is generated in the direction to separate the mutually engaging gears from each other (hereinafter referred to as “gear separating force”, for the sake of explanation). As the rotational speeds of the gears become higher, the gear separating force increases. Grooves 110 are formed in side plates at the both sides of the gears 104, 107. The oil at the discharge side flows into the clearance between the gears 104, 107 and the housing 102 through the grooves 110, and urging force is generated in the direction to move the gears 104, 107 close to each other (hereinafter referred to as “gear approaching force”, for the sake of explanation). Grooves 111 are formed in side plates at the both sides of the gears 108, 109, and the oil at the discharge side flows into the clearance between the gears 108, 109 and the housing 102 through the grooves 111, and the gear approaching force is generated in the direction to move the gears 108, 109 close to each other.

Following will explain the case where the gear pump 100 of the background art in FIG. 7 is utilized as a variable displacement type gear pump. The driven gear 107 is formed independently of the driven gear 109. Thereby the driven gear 109 in the pump mechanism Pa is moved to and away from the drive gear 108 within the range of the clearance between the bush 106 and the gear 109, when the gear separating force is generated in small displacement operation. The separation of the driven gear 109 is caused in such a manner that the gear separating force generated by the pressure increase in the trap region is applied to the driven gear 109. The approach of the driven gear 109 is caused in such a manner that the engagement of the teeth trapping the highly-pressurized oil in the trap region is released, and that the gear separating force is released to return the driven gear 109 to the position before the separation. When the driven gear 109 is rotated with high speed, the cycle of the separation and the approach is reduced while the gear separating force is increased. Such separation and approach of the driven gear may cause abnormal noise and vibration during the operation of the gear pump.

Similar to the pump mechanism Pa, the pressure of the oil is increased in the trap region defined by the gears 104, 107 of the pump mechanism Pb. However, the pressure at the discharge side of the pump mechanism Pb is high even in the small displacement operation, and gear approaching force is generated to move the gears 104, 107 close to each other. The gear approaching force is great relative to the gear separating force which is based on the oil pressure in the trap region. Therefore the effect of the gear separating force can be neglected.

The present invention is directed to provide a variable displacement type gear pump that prevents separation and approach of a specific gear to a gear engaging with the specific gear in small displacement operation.

SUMMARY OF THE INVENTION

In accordance with the present invention, a variable displacement type gear pump introduces and discharges fluid. The gear pump includes a main gear pump portion, a sub gear pump portion, a suction-side space, a suction passage, a discharge-side space, an outlet passage, a bypass passage, an opening and closing valve, a check valve, a drive shaft, and a driven shaft. The main gear pump portion has a main drive gear, a main driven gear, and a main urging portion. The main drive gear and the main driven gear are engaged with each other and applied a radial load by the main urging portion. The sub gear pump portion has a sub drive gear, a sub driven gear, and a sub urging portion. The sub drive gear and the sub driven gear are engaged with each other and applied a radial load by the sub urging portion. The suction-side space is formed in the main gear pump portion and the sub gear pump portion, respectively. The suction passage communicates with the suction-side spaces. The discharge-side space is formed in the main gear pump portion and the sub gear pump portion, respectively. The outlet passage communicates with the discharge-side spaces. The bypass passage returns the fluid in the discharge-side space of the sub gear pump portion to the suction passage. The opening and closing valve is formed in the bypass passage and opens the bypass passage in a small displacement operational state. The check valve is located between the discharge-side space of the sub gear pump portion and the outlet passage for preventing the fluid discharged from the main gear pump portion from flowing into the discharge-side space of sub gear pump portion in the small displacement operational state. The drive shaft supports the main drive gear and the sub drive gear. The driven shaft supports the main driven gear and the sub driven gear. The main and sub drive gears and the main and sub driven gears are rotated in accordance with a rotation of the drive shaft. The driven and driven shafts transmit the radial load in the main gear pump portion to the sub gear pump portion in the small displacement operational state.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

FIG. 1 is a longitudinal cross-sectional view of a variable displacement type gear pump according to a preferred embodiment of the present invention;

FIG. 2A is a cross-sectional view of the gear pump taken along the line I-I in FIG. 1;

FIG. 2B is a side view showing a side plate of the gear pump according to the preferred embodiment;

FIG. 3 is a cross-sectional view of the gear pump taken along the line II-II in FIG. 2A;

FIG. 4 is a cross-sectional view of the gear pump taken along the line III-III in FIG. 1;

FIG. 5 is a cross-sectional view of the gear pump in small displacement operational state;

FIG. 6 is a cross-sectional view of a variable displacement type gear pump according to a modified embodiment;

FIG. 7 is a cross-sectional view of a gear pump as a background art; and

FIG. 8 is a schematic view showing a hydraulic circuit utilizing the gear pump of FIG. 7 as a variable displacement type gear pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment according to the present invention will be described with reference to FIGS. 1 through 5. FIG. 1 shows a variable displacement type gear pump in large displacement operational state.

A variable displacement type gear pump 10 as shown in FIGS. 1 through 5 (hereinafter referred to merely as a gear pump) includes a body 11 which accommodates a main drive gear 22, a sub drive gear 25, a main driven gear 23, and a sub driven gear 26 therein. The body 11 has two spaces which are formed so as to extend from both axial end faces of the body 11. One space is a main gear chamber 12 and the other space is a sub gear chamber 13. A partition 14 is formed between the main gear chamber 12 and the sub gear chamber 13.

A front housing 15 is joined to one end face of the body 11. A rear housing 16 is joined to the other end face of the body 11. In this embodiment, the body 11, the front housing 15, and the rear housing 16 constitute a housing assembly. The body 11 and the front and the rear housings 15, 16 are joined by through bolts 30 with each other as shown in FIGS. 2A and 4. It is noted that the front side where the front housing 15 is located corresponds to the left side in the drawings (referring to FIGS. 1, 3, and 5). Similarly, the rear side where the rear housing 16 is located corresponds to the right side in the drawings. The front housing 15 closes the main gear chamber 12, and the rear housing 16 closes the sub gear chamber 13.

A side plate 17 is formed adjacent to the main gear chamber 12 so as to be interposed between the main gear chamber 12 and the end face of the front housing 15. A side plate 18 is formed adjacent to the sub gear chamber 13 so as to be interposed between the sub gear chamber 13 and the end face of the rear housing 16. Similarly, a side plate 19 is formed adjacent to the main gear chamber 12 so as to be interposed between the main gear chamber 12 and the partition 14. A side plate 20 is formed adjacent to the sub gear chamber 13 so as to be interposed between the sub gear chamber 13 and the partition 14. FIG. 2B shows the configuration of the side plate 17. A groove 17A as a main urging portion is formed in the side plate 17 to urge the gears 22, 23 to each other, as described later. Similarly, a groove 19A as an another main urging portion is formed in the side plate 19, and grooves 18A, 20A as sub urging portions are formed in the side plates 18, 20, as shown in FIG. 3.

The main drive gear 22 and the main driven gear 23 are externally engaged with each other so as to form a main gear mechanism 21. The main gear mechanism 21 is accommodated in the main gear chamber 12 as shown in FIGS. 2A and 3. The sub drive gear 25 and the sub driven gear 26 are externally engaged with each other so as to form a sub gear mechanism 24. The sub gear mechanism 24 is accommodated in the sub gear chamber 13 as shown in FIGS. 3 and 4. The main drive gear 22 accommodated in the main gear chamber 12 is formed integrally with a drive shaft 27. The main drive gear 22 is coaxial with the drive shaft 27. The sub drive gear 25 accommodated in the sub gear chamber 13 has a through hole 25A through which the drive shaft 27 is inserted.

The drive shaft 27 has a spline 27A formed at the position corresponding to the sub drive gear 25. The through hole 25A has a spline groove corresponding to the spline 27A. The drive shaft 27 and the sub drive gear 25 are connected by way of spline coupling as shown in FIG. 4. In this embodiment, the spline 27A, the through hole 25A having the spline groove constitute a rotational force transmitting device for transmitting the rotational force of the drive shaft 27 to the sub drive gear 25. The sub drive gear 25 is coaxial with the drive shaft 27. That is, the main drive gear 22 and the sub drive gear 25 have a common rotational axis. In the above-described manner, the drive shaft 27 supports the main drive gear 22 and the sub drive gear 25.

The drive shaft 27 extends through the side plates 17, 18, 19, 20 and the partition 14 into the front and rear housings 15, 16. The drive shaft 27 is rotatably supported by the body 11, the front and rear housings 15, 16 through a bearing 29. One end of the drive shaft 27 extends out of the front housing 15 so as to be connected to an external drive source which is not shown, and receives driving force from the external drive source.

Next will describe the main driven gear 23 and the sub driven gear 26. The main driven gear 23 is formed integrally with a driven shaft 28, and is coaxial with the driven shaft 28. As shown in FIG. 4, the sub driven gear 26 has a through hole 26A in which the driven gear 28 is rotatably inserted. The through hole 26A has a circular cross section. The driven shaft 28 has an insert shaft portion 28A at a position corresponding to the sub driven gear 26. The insert shaft portion 28A is formed so as to correspond to the through hole 26A of the driven gear 26. In the state where the driven shaft 28 is inserted in the through hole 26A, the driven shaft 28 and the sub driven gear 26 are relatively rotatable and relatively movable in the axial direction. The clearance between the insert shaft portion 28A and the through hole 26A is set minimum so as to permit the relative rotation and relative axial movement of the driven shaft 28 and the sub driven gear 26. That is, the driven shaft 28 has the insert shaft portion 28A which is not press-fitted but is closely inserted into the through hole 26A. In the state where the driven shaft 28 is inserted in the through hole 26A, the sub driven gear 26 is coaxial with the driven shaft 28. In the above-described manner, the driven shaft 28 supports the main driven gear 23 and the sub driven gear 26.

Similar to the drive shaft 27, the driven shaft 28 extends into the front housing 15 and the rear housing 16. The driven shaft 28 is supported by the body 11, the front housing 15 and the rear housing 16 through an another bearing 29. The clearance between the insert shaft portion 28A and the through hole 26A is set small so as to merely permit the relative rotation and the relative axial movement of the driven shaft 28 and the sub driven gear 26. That means the clearance is almost in the level equivalent to no substantial clearance, compared to, for example, the amount of the clearance between the bearing 29 and the driven shaft 28. The main driven gear 23 and the sub driven gear 26 have a common rotational axis. Unlike the drive shaft 27, the end of the driven shaft 28 does not extend out of the front housing 15.

As shown in FIG. 2A, the main gear chamber 12 includes a suction-side space 31 and a discharge-side space 32 defined by the inner surface of the main gear chamber 12, the main drive gear 22, and the main driven gear 23. The suction-side space 31 is formed at the suction side for introducing oil as a fluid. The discharge-side space 32 is formed at the discharge side for discharging the oil. Similar to the main gear chamber 12, a suction-side space 33 and a discharge-side space 34 are defined in the sub gear chamber 13 (as shown in FIGS. 1 and 4).

As shown in FIG. 1, the main gear mechanism 21, the side plates 17, 19, and the main gear chamber 12 constitute a main gear pump portion P1. The sub gear mechanism 24, the side plates 18, 20, and the sub gear chamber 13 constitute a sub gear pump portion P2. The main gear pump portion P1 and the sub gear pump portion P2 respectively have 50 percent of the entire discharge displacement of the variable displacement type gear pump 10.

A front suction passage 36 is formed in the body 11 along the axial direction of the drive shaft 27 and the driven shaft 28. The oil is introduced into the main and the sub gear chambers 12, 13 through the front suction passage 36. A rear suction passage 37 is formed in the rear housing 16 and communicates with the front suction passage 36. The rear suction passage 37 has an inlet 38 which is open at the axial end face of the rear housing 16. The inlet 38 is in communication with the outside. The front suction passage 36 and the rear suction passage 37 have circular cross-section, and are linearly connected with each other. The front suction passage 36 and the rear suction passage 37 constitute a suction passage 35. The suction passage 35 is communicated with the suction-side spaces 31, 33. The oil from the outside of the gear pump 10 flows into the main and the sub gear chambers 12, 13 through the suction passage 35.

A front discharge passage 42 and a rear discharge passage 43 are formed in the body 11 for discharging the oil pressurized in the main and sub gear chambers 12, 13 to the outside. The front discharge passage 42 extends from the discharge-side space 32 of the main gear chamber 12. The rear discharge passage 43 extends from the discharge-side space 34 of the sub gear chamber 13. The front and rear discharge passages 42, 43 are connected with each other so as to communicate with a single outlet passage 41 downstream thereof inside the body 11. The outlet passage 41 communicates with the discharge-side spaces 32, 34. Further, the outlet passage 41 has an outlet 44 which is in communication with the outside. The oil is discharged to the outside of the gear pump 10 through the outlet passage 41 and the outlet 44. Then the oil is delivered to a hydraulic circuit connected to a hydraulic device, which is not shown. A check valve 45 is provided in the rear discharge passage 43 in the body 11 and is located between the discharge-side space 34 of the sub gear pump portion P2 and the outlet passage 41. The check valve 45 serves to prevent the oil discharged from the main gear pump portion P1 from flowing into the discharge-side space 34 of the sub gear pump portion P2 in a small displacement operational state.

The check valve 45 includes a ball-shaped valve body 46, a coil spring 47, and a support member 48. The valve body 46 opens and closes the rear discharge passage 43. The coil spring 47 is a urging device for urging the valve body 46. The support member 48 supports the coil spring 47. The coil spring 47 applies urging force to the valve body 46 in the direction to close the rear discharge passage 43. The valve body 46 is moved in the direction to open the rear discharge passage 43 against the urging force of the coil spring 47 when the pressure in the rear discharge passage 43 becomes equal or greater than a predetermined value. The valve body 46 closes the rear discharge passage 43 by the urging force of the coil spring 47 when the pressure in the rear discharge passage 43 decreases below the predetermined value. The urging force of the coil spring 47 may be set small, since the valve body 46 is urged to a seat surface in the support member 48 by the pressure difference. The form of the valve body 46 is not limited to the ball-shape, and may be a conical shape.

The rear housing 16 has a bypass passage 50 which is in communication with the rear discharge passage 43, and also in communication with the rear suction passage 37. That is, the bypass passage 50 is in communication with the suction passage 35 and the discharge-side space 34 of the sub gear chamber 13. An opening and closing valve 51 is formed in the bypass passage 50 to open and close the bypass passage 50. It is noted that part of the bypass passage 50 upstream of the valve 51 is defined as an upstream-side passage 50A. Similarly, part of the bypass passage 50 downstream of the valve 51 is defined as a downstream-side passage 50B. The valve 51 has a piston mechanism in which a cylindrical piston 53 is slidably accommodated in a bottomed hollow cylinder 52.

The valve 51 opens and closes the bypass passage 50 by the sliding movement of the piston 53 in the cylinder 52. The sliding movement of the piston 53 is performed by the pressure difference applied to the opposite end faces of the piston 53. That is, the sliding movement of the piston 53 is performed by the pressure difference between the pressure applied to the end face facing the bypass passage 50 and the pressure in the cylinder 52 applied to the opposite end face of the piston 53.

In this embodiment, the pressure difference applied on the opposite end faces of the piston 53 is controlled by the actuation of an electromagnetic valve 55 in the rear housing 16. The electromagnetic valve 55 includes a spool 57, a coil 58, and a coil spring 59. The spool 57 slides in a spool hole 56 formed in the rear housing 16. The coil 58 moves the spool 57 frontward. The coil spring 59 is an urging device for urging the spool 57. The spool hole 56 is in communication with the downstream-side passage 50B of the bypass passage 50. The spool 57 includes a suction-pressure communication passage 60 for communicating the bypass passage 50 to the cylinder 52. When the coil 58 is excited, the spool 57 is moved frontward. When the coil 58 is de-excited, the spool 57 is moved rearward by the coil spring 59.

A discharge-pressure communication passage 61 is formed in the body 11 and the rear housing 16 for supplying oil under discharge pressure from the rear discharge passage 43 to the spool hole 56. The discharge-pressure communication passage 61 includes passages 62, 63 and a groove 64. The groove 64 is formed at the outer periphery of the spool 57. When the coil 58 is de-excited and the spool 57 is in the rear position, the discharge pressure in the rear discharge passage 43 is applied to the cylinder 52 through the discharge-pressure communication passage 61. When the coil 58 is exited and the spool 57 is in the front position, the suction-pressure communication passage 60 releases oil under discharge pressure in the cylinder 52 to the bypass passage 50. When the pressure in the cylinder 52 becomes to suction pressure due to the excitation of the coil 58, the piston 53 opens the bypass passage 50. When the pressure in the cylinder 52 becomes to discharge pressure due to the de-excitation of the coil 58, the piston 53 closes the bypass passage 50.

The following will describe the operation of the variable displacement type gear pump 10 of the preferred embodiment according to the present invention. Firstly, the operation of the main drive gear 22 and the main driven gear 23 in the main gear pump portion P1 will be explained. When the driving force is applied to the drive shaft 27 from the outside, the main drive gear 22 is rotated in one direction as shown in FIG. 2A. According to the rotation of the main drive gear 22, the main driven gear 23 engaging with the main drive gear 22 is rotated with the driven shaft 28 in the direction opposite to the rotational direction of the main drive gear 22. When the main drive gear 22 and the main driven gear 23 are rotated while being engaged with each other, oil is introduced into the suction-side space 31 from the suction passage 35.

When the oil is introduced into the suction-side space 31, the oil is confined in a space defined by teeth of the main drive gear 22 and the inner surface of the main gear chamber 12, and also in a space defined by teeth of the main driven gear 23 and the inner surface of the main gear chamber 12. The oil confined in the spaces is transferred along the inner surface of the main gear chamber 12 in the rotational direction of the main drive gear 22 and the rotational direction of the main driven gear 23, respectively. The oil confined in the spaces is discharged to the discharge-side space 32. The oil in the discharge-side space 32 is discharged to the outside of the gear pump 10 through the front discharge passage 42, the outlet passage 41, and the outlet 44, and delivered to a hydraulic device not shown to operate the hydraulic device. The discharge pressure is increased in accordance with the load in the hydraulic device.

In the main gear pump portion P1, when the driving force is applied to the drive shaft 27 from the outside, the main drive gear 22 and the main driven gear 23 are driven in the main gear chamber 12, and the oil is discharged to the discharge-side space 32. The discharged oil is supplied to the front discharge passage 42. In the sub gear pump portion P2, when the driving force is applied to the drive shaft 27 from the outside, the sub drive gear 25 and the sub driven gear 26 are driven in the sub gear chamber 13, and the oil is discharged to the discharge-side space 34.

When the coil 58 of the electromagnetic valve 55 is not excited, the spool 57 is located in the rear position, by receiving the urging force of the coil spring 59. When the spool 57 is located in the rear position, the discharge-pressure communication passage 61 is communicated with the cylinder 52 of the valve 51. The communication between the suction-pressure communication passage 60 and the cylinder 52 is shut off. Therefore, oil is introduced from the rear discharge passage 43 through the discharge-pressure communication passage 61, and the cylinder 52 is filled with the oil under the discharge pressure. In the state where the piston 53 does not close the bypass passage 50 yet, the pressure in the bypass passage 50 communicating with the suction passage 35 is lower than the pressure in the cylinder 52, and the piston 53 is moved in the direction to close the bypass passage 50.

When the piston 53 closes the bypass passage 50, the pressure in the discharge-side space 34 receiving the oil discharged from the sub gear pump portion P2 is increased, and the valve body 46 of the check valve 45 opens the rear discharge passage 43. Thereby the oil discharged from the sub gear pump portion P2 is supplied to the rear discharge passage 43 and the bypass passage 50. As shown in FIG. 1, the oil flows from the rear discharge passage 43 to the outlet passage 41, and does not flow to the suction passage 35 through the bypass passage 50. In this state, the oil discharged from the main gear pump portion P1 and the sub gear pump portion P2 joins together, and is delivered to the outside of the gear pump 10 through the outlet passage 41. Therefore, in this state, the discharge displacement of the variable displacement type gear pump 10 is 100 percent, and the gear pump 10 is in the large displacement operational state. When the gear pump 10 is utilized in a material handling device of a forklift truck, the large displacement operational state of 100 percent may be set to correspond to the load lifting-up state. The piston 53 has a larger diameter at the side adjacent to the cylinder 52 (rear side) than the front side. Thereby the piston 53 is reliably urged frontward to close the bypass passage 50, even when the pressures at the front and rear sides of the piston 53 are equal with each other.

When the coil 58 of the electromagnetic valve 55 is excited, the spool 57 receives the frontward force overcoming the urging force of the coil spring 59, and is moved frontward. When the spool 57 is located in the front position, the communication between the discharge-pressure communication passage 61 and the cylinder 52 is shut off. In this state, the spool hole 56, the suction-pressure communication passage 60, and the cylinder 52 are communicated with each other. Thereby the pressure in the cylinder 52 of the valve 51 is decreased from the discharge pressure to the suction pressure. Since the upstream side of the bypass passage 50 equals to the discharge pressure, the piston 53 is moved into the cylinder 52 by receiving the pressure difference when the pressure in the cylinder 52 becomes the suction pressure. By the retreat of the piston 53 into the cylinder 52, the bypass passage 50 becomes to the fully opened state.

Since the pressure at the upstream side of the bypass passage 50 is decreased, the valve body 46 closes the rear discharge passage 43 by the urging force of the coil spring 47 of the check valve 45. As shown in FIG. 5, when the check valve 45 closes the rear discharge passage 43 and the valve 51 opens the bypass passage 50, only the oil discharged from the main gear pump portion P1 is discharged to the outside through the outlet passage 41. The oil discharged from the sub gear pump portion P2 is supplied to the bypass passage 50. Then the oil joins the upstream side of the suction passage 35. Therefore, in this state, the discharge displacement of the variable displacement type gear pump 10 gets to 50 percent, and is in the small displacement operational state. In this embodiment, the state where the electromagnetic valve 55 is activated is set as the 50-percent discharge displacement. However, the location of the groove 64 and the suction-pressure communication passage 60 in the spool 57 may be modified so that a state activating the electromagnetic valve 55 is set as 100-percent discharge displacement, while a state deactivating the electromagnetic valve 55 is set as 50-percent discharge displacement.

The main drive gear 22 and the sub drive gear 25 are rotated integrally with the drive shaft 27 in accordance with the rotation of the drive shaft 27. Since the main drive gear 22 is formed integrally with the drive shaft 27, the rotation of the drive shaft 27 corresponds directly to the rotation of the main drive gear 22. On the other hand, the sub drive gear 25 is connected to the drive shaft 27 by way of spline coupling, and the rotation of the drive shaft 27 is transmitted to the sub drive gear 25 by the rotational force transmitting device including the spline 27A and the through hole 25A.

The main driven gear 23 and the sub driven gear 26 are rotated integrally with the driven shaft 28 in accordance with the rotation of the drive shaft 27. Since the main driven gear 23 is formed integrally with the driven shaft 28, the rotation of the driven shaft 28 corresponds directly to the rotation of the main driven gear 23. On the other hand, the insert shaft portion 28A of the driven shaft 28 is inserted in the through hole 26A with the circular cross section in the sub driven gear 26, and the rotation of the driven shaft 28 is not transmitted to the sub driven gear 26. The sub driven gear 26 receives the rotational force of the drive shaft 27 through the engagement to the sub drive gear 25. Though the rotation of the drive shaft 27 is transmitted independently to the driven shaft 28 and the sub driven gear 26, the driven shaft 28 and the sub driven gear 26 are rotated synchronously with little peripheral speed difference.

In the operation of the gear pump 10, the oil is highly pressurized in the trap region formed by the gears 22, 23, and in the trap region formed by the gears 25, 26, due to the engagement. The pressure of the oil in the trap regions generates gear separating force to separate the main driven gear 23 from the main drive gear 22, and also gear separating force to separate the sub driven gear 26 from the sub drive gear 25. The grooves 17A, 19A of the side plates 17, 19 are in communication with the discharge-side space 32 as shown in FIG. 1. Thereby the pressurized oil in the discharge-side space 32 is introduced into the clearance between the gears 22, 23 and the main gear chamber 12 through the grooves 17A, 19A, and generates gear approaching force to move the gears 22, 23 close to each other. In this embodiment, the grooves 17A, 19A serve as the main urging portions to urge the gears 22, 23 by applying the gear approaching force as a radial load. Similarly, the grooves 18A, 20A of the side plates 18, 20 are in communication with the discharge-side space 34 as shown in FIG. 1, and generates gear approaching force to move the gears 25, 26 close to each other. The grooves 18A, 20A serve as the sub urging portions to urge the gears 25, 26 by applying the gear approaching force as a radial load.

In the large displacement operation, the pressures in the discharge-side spaces 32, 34 are high. Thereby the gear approaching forces are capable of sufficiently resisting against the gear separating forces which are applied to separate the drive gear 22 from the driven gear 23 in the main gear mechanism 21, and also the driven gear 26 from the drive gear 25 in the sub gear mechanism 24. Therefore, the drive shaft 27 and the driven shaft 28 are positioned within the range of the clearances to the bearings 29, by receiving the gear approaching forces in the main and sub gear mechanisms 21, 24.

In the small displacement operational state, since the pressure in the discharge-side space 32 is large in the main gear mechanism 21, the effect of the gear separating force is negligible, similar to the case in the large displacement operation. The rotating drive shaft 27 and the driven shaft 28 are positioned in the range of the clearances to the bearings 29, by receiving the gear approaching force in the main gear mechanism 21, similar to the case in the large displacement operation. On the other hand, in the sub gear pump portion P2, the pressure in the discharge-side space 34 is lower than the pressure in the large displacement operation. Therefore, the gear approaching force in the sub gear mechanism 24 is small, so that the effect of the gear separating force in the sub gear mechanism 24 becomes relatively high. However, the main driven gear 23 is formed integrally with the driven shaft 28, and the driven shaft 28 is inserted in the sub driven gear 26. Thereby the gear approaching force applied to the main drive gear 22 and the main driven gear 23 is applied to the sub drive gear 25 and the sub driven gear 26 through the drive shaft 27 and the driven shaft 28, and restricts the separation of the sub drive gear 25 and the sub driven gear 26. That is, the drive shaft 27 and the driven shaft 28 serve to restrict the separation of the sub drive gear 25 from the sub driven gear 26 in the sub gear pump portion P2.

In the small displacement operational state, separation and approach of the sub driven gear 26 is repeated in the range of the clearance between the through hole 26A and the insert shaft portion 28A. However, the range of the separation and approach of the sub driven gear 26 is much smaller than the clearance between the bearing 29 and the driven shaft 28, and in the negligible level in the aspect of noise and vibration. Thus, the driven shaft 28 restricts the separation of the sub driven gear 26, opposing to the gear separating force in the sub gear mechanism 24, since the driven shaft 28 connects the driven gears 23, 26 so as to transmit radial load. In this embodiment, the gear approaching force applied to the main drive gear 22 and the main driven gear 23 serves as the radial load, and is transmitted from the main gear mechanism 21 of the main gear pump portion P1 to the sub gear mechanism 24 of the sub gear pump portion P2 through the drive shaft 27 and the driven shaft 28. Accordingly, in the small displacement operation, the separation and approach of the sub driven gear 26 with respect to the engaging sub drive gear 25 is prevented in the sub gear pump portion P2.

The variable displacement type gear pump 10 of the preferred embodiment according to the present invention has the following advantageous effects.

(1) During the small displacement operation of the gear pump 10, the gear approaching force is small and the gear separating force becomes relatively influential in the sub gear mechanism 24. However, the gear approaching force to the main gears 22, 23 is applied to the sub gears 25, 26 through the drive and driven shafts 27, 28. Further, even if the gear separating force is generated in the sub gear mechanism 24 of the sub gear pump portion P2, the repetition of separation and approach of the sub driven gear 26 is restricted in the range of the clearance between the through hole 26A and the insert shaft portion 28A. The separation and approach of the sub driven gear 26 is in the negligible range in the aspect of noise and vibration. Therefore, in the small displacement operational state, the separation and approach of the sub driven gear 26 with respect to the engaging sub drive gear 25 is prevented in the sub gear pump portion P2.

(2) The driven shaft 28 is relatively rotatable to the sub driven gear 26. Thereby, in assembling the gear pump 10, the sub driven gear 26 is positioned with respect to the sub drive gear 25 as easily as the background art, after engaging the main driven gear 23 to the main drive gear 22 in the main gear pump portion P1.

A modified embodiment to the above preferred embodiment is shown in FIG. 6. Like or same parts or elements will be referred to by the same reference numerals as those in the above embodiment.

The driven shaft 28 and the sub driven gear 26 may be connected by way of spline coupling, similar to the spline coupling of the drive shaft 27 and the sub drive gear 25. In this modified embodiment, the rotation of the driven shaft 28 is transmitted to the sub driven gear 26 by the spline coupling between the driven shaft 28 and the sub driven gear 26. The rotation of the drive shaft 27 is also transmitted to the sub driven gear 26 through the engagement of the sub drive gear 25 and the sub driven gear 26.

In the modified embodiment, the separation and approach of the sub driven gear 26 with respect to the engaging sub drive gear 25 is prevented in the sub gear pump portion P2 during the small displacement operation, similar to the above preferred embodiment. In the modified embodiment, high machining accuracy is required for positioning the main driven gear 23 and the sub driven gear 26 to the main drive gear 22 and the sub drive gear 25 for mutual engagement.

The above-described embodiments including the modified embodiment are to be considered as illustrative and not restrictive. Therefore, the present invention is not limited to the above-described embodiments but modifications are applicable as follows within the scope of the invention.

In the preferred embodiment, when the maximum displacement of the variable displacement type gear pump is set as 100 percent, and the displacement of each of the main and sub gear pump portions is set as 50 percent. The performance of the each gear pump portions may be set appropriately, for example, as 70 percent and 30 percent, depending on the condition.

In the preferred embodiment, two gear pump portions, that is, the main gear pump portion and the sub gear pump portion are provided. However, the number of gear pump portions may be more than two. In this case, the oil discharged from at least one gear pump portion may flow through the bypass passage in the small displacement operation.

In the preferred embodiment, the suction passage has the circular cross section over the entire longitudinal direction. However, the suction passage may not have a circular cross section. The cross section of the suction passage may be, for example, polygonal, elliptical, or oblong shape.

In the preferred embodiment, the discharge passages are provided so as to connect the discharge-side space of each gear chamber and the outlet passage. The discharge-side space of each gear chamber may be connected directly to the outlet passage without a discharge passage. In this case, a check valve is required to shut off the outlet passage which connects the gear chambers.

In the preferred embodiment, the bypass passage is formed so as to pass through the rear side of the rear ends of the drive shaft and the driven shaft. However, the bypass passage is not limited to be formed in the above-described position. For example, the bypass passage may pass around at least one of the outer periphery of the drive shaft and the driven shaft. In this case, the bypass passage may be preferably formed between the rearmost gear chamber and the rear ends of the drive shaft and the driven shaft, in order to form the confluence portion of the bypass passage to the suction passage at the upstream side of the suction passage.

In the preferred embodiment, the sub driven gear has the through hole with the circular cross section, and the driven shaft is inserted in the through hole thereby allowing the relative rotation and the relative axial movement of the driven shaft and the sub driven gear. The sub driven gear may be formed integrally with the driven shaft, similar to the main driven gear. In this case, a through hole is not formed in the sub driven gear and clearance does not exist between the driven shaft and the sub driven gear. Therefore, the separation and approach of the sub driven gear is more reliably prevented in the small displacement operation, compared to a case having a through hole. However, high machining accuracy is required to engage the main driven gear and sub driven gear respectively to the main drive gear and the sub drive gear. The driven shaft may be inserted in the main driven gear and the sub driven gear. In this case, the machining process of the parts is easily performed. The main driven gear and the sub driven gear may be formed integrally with different driven shafts, respectively, and the driven shafts may be connected by screw portions formed at each ends of the driven shafts. In this case, the driven gears are relatively rotatable, and the engaging process of the gears is performed easily in assembling the gear pump.

In the preferred embodiment, the spline coupling is utilized as the rotational force transmitting device between the drive shaft and the sub drive gear, but the rotational force transmitting device is not limited to the spline coupling. The rotational force transmitting device may be a key or serration. Alternatively, the rotational force transmitting device may be a screw mechanism fixing the sub drive gear to the drive shaft by screwing. The rotational force transmitting device may be the structure where the sub drive gear is formed integrally with the drive shaft. Further, the rotational force transmitting device may be utilized between the driven shaft and the sub driven gear.

Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims. 

1. A variable displacement type gear pump for introducing and discharging fluid, comprising: a main gear pump portion having a main drive gear, a main driven gear, and a main urging portion, wherein the main drive gear and the main driven gear are engaged with each other and applied a radial load by the main urging portion; a sub gear pump portion having a sub drive gear, a sub driven gear, and a sub urging portion, wherein the sub drive gear and the sub driven gear are engaged with each other and applied a radial load by the sub urging portion; a suction-side space formed in the main gear pump portion and the sub gear pump portion, respectively; a suction passage communicating with the suction-side spaces; a discharge-side space formed in the main gear pump portion and the sub gear pump portion, respectively; an outlet passage communicating with the discharge-side spaces; a bypass passage returning the fluid in the discharge-side space of the sub gear pump portion to the suction passage; an opening and closing valve formed in the bypass passage and opening the bypass passage in a small displacement operational state; a check valve located between the discharge-side space of the sub gear pump portion and the outlet passage for preventing the fluid discharged from the main gear pump portion from flowing into the discharge-side space of the sub gear pump portion in the small displacement operational state; a drive shaft supporting the main drive gear and the sub drive gear; and a driven shaft supporting the main driven gear and the sub driven gear, wherein the main and sub drive gears and the main and sub driven gears are rotated in accordance with a rotation of the drive shaft, wherein the drive and driven shafts transmit the radial load in the main gear pump portion to the sub gear pump portion in the small displacement operational state.
 2. The variable displacement type gear pump according to claim 1, wherein the main drive gear is formed integrally with the drive shaft.
 3. The variable displacement type gear pump according to claim 2, wherein the sub drive gear is connected to the drive shaft through a rotational force transmitting device so that the rotational force transmitting device transmits the rotation of the drive shaft to the sub drive gear.
 4. The variable displacement type gear pump according to claim 3, wherein the sub drive gear has a through hole with a spline groove formed therein, wherein the drive shaft has a spline formed therein so that the sub drive gear is connected to the drive shaft by spline coupling, wherein the rotational force transmitting device includes the spline formed in the drive shaft and the spline groove formed in the through hole of the sub drive gear.
 5. The variable displacement type gear pump according to claim 2, wherein the main driven gear is formed integrally with the driven shaft.
 6. The variable displacement type gear pump according to claim 5, wherein the driven shaft and the sub driven gear are relatively rotatable.
 7. The variable displacement type gear pump according to claim 6, wherein the sub driven gear has a through hole with a circular cross section in which the driven shaft is rotatably inserted.
 8. The variable displacement type gear pump according to claim 7, wherein clearance between the through hole of the sub driven gear and the driven shaft is set smaller than the clearance between the driven shaft and a bearing for the driven shaft.
 9. The variable displacement type gear pump according to claim 1, wherein the main drive gear is formed integrally with the drive shaft, wherein the sub drive gear has a through hole in which the drive shaft is inserted, wherein rotational force of the drive shaft is transmitted to the sub drive gear through a rotational force transmitting device, wherein the main driven gear is formed integrally with the driven shaft, wherein the sub driven gear has a through hole with a circular cross section, wherein the driven shaft is rotatably inserted in the through hole.
 10. The variable displacement type gear pump according to claim 1, the main gear pump portion further comprising: a main gear chamber; a main gear mechanism formed in the main gear chamber; and a side plate formed adjacent to the main gear chamber, wherein the side plate has a groove as the main urging portion for introducing the fluid of the discharge-side space and generating a gear approaching force as the radial load in the main gear pump portion to move the main drive gear and the main driven gear close to each other, and wherein the gear approaching force in the main gear pump portion is transmitted to the sub gear pump portion through the drive and driven shafts in the small displacement operational state. 